Method and apparatus for a geometric aberration transform in an adaptive focusing ultrasound beamformer system

ABSTRACT

A method and an apparatus are provided for storage and retrieval of measured aberration correction values used in adaptive focus ultrasound imaging. The aberration correction values, corresponding to an aberration region in an imaged subject, are measured at a transmit focal depth. The measured aberration correction values, typically delays, from a measurement depth are stored in a geometric aberration transform (GAT™) table and transformed to aberration correction values at other focal depths for correcting the focus of both transmit and receive beamformers. The transformation is accomplished by using a geometric aberration transform (GAT™) index table which retrieves one or more aberration correction values in the table for any desired depth.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is related to the following application all filed onAug. 5, 1994:

    __________________________________________________________________________    Title            Inventors Atty. Docket No.                                                                          Ser. No.                               __________________________________________________________________________    METHOD AND APPARATUS                                                                           J. Nelson Wright                                                                        ACUS-1000 SRM                                                                             08/286,658                             FOR RECEIVE      Christopher R. Cole                                          BEAMFORMER SYSTEM                                                                              Albert Gee                                                   METHOD AND APPARATUS                                                                           Christopher R. Cole                                                                     ACUS-1001 SRM/WSW                                                                         08/286,652                             FOR TRANSMIT     Albert Gee                                                   BEAMFORMER SYSTEM                                                                              Thomas Liu                                                   METHOD AND APPARATUS                                                                           Albert Gee                                                                              ACUS-1002 SRM                                                                             08/286,268                             FOR FOCUS CONTROL OF                                                                           Christopher R. Cole                                          TRANSMIT AND RECEIVE                                                                           J. Nelson Wright                                             BEAMFORMER SYSTEMS                                                            METHOD AND APPARATUS                                                                           Samuel H. Maslak                                                                        ACUS-1003 SRM/WSW                                                                         08/286,648                             FOR DOPPLER RECEIVE                                                                            Christopher R. Cole                                          BEAMFORMER SYSTEM                                                                              Joseph G. Petrofsky                                          METHOD AND APPARATUS                                                                           J. Nelson Wright                                                                        ACUS-1004 SRM/KJD                                                                         08/286,528                             FOR REAL-TIME,   Samuel H. Maslak                                             CONCURRENT ADAPTIVE                                                                            Donald R. Langdon                                            FOCUSING IN AN   Gregory L. Holley                                            ULTRASOUND       Christopher R. Cole                                          BEAMFORMER IMAGING                                                            SYSTEM                                                                        METHOD AND APPARATUS                                                                           J. Nelson Wright                                                                        ACUS-1006 SRM                                                                             08/286,510                             FOR COHERENT IMAGE                                                                             Samuel H. Maslak                                             FORMATION        David J. Finger                                                               Albert Gee                                                   METHOD AND APPARATUS                                                                           J. Nelson Wright                                                                        ACUS-1022 SRM/WSW                                                                         08/286,524                             FOR ADJUSTABLE   Christopher R. Cole                                          FREQUENCY SCANNING IN                                                                          Albert Gee                                                   ULTRASOUND IMAGING                                                                             Hugh G. Larsen                                                                Samuel H. Maslak                                             __________________________________________________________________________

The related patent applications are all commonly assigned with thepresent application, filed concurrently with the present application,and are all incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to ultrasound imaging and, in particular,to adaptive focusing ultrasound imaging systems which provide aberrationcorrection values for distorted ultrasound beams caused by an aberratingregion.

BACKGROUND OF THE INVENTION

A. Description of the Related Art

Ultrasound imaging systems use time delays and/or phase rotation meansto form focused ultrasound beams. On transmit, time delays and/or phaserotation means are used to bring ultrasound pulses from differenttransducer elements to the desired focal point with temporal alignmentand phase coherence. Likewise, on receive, time delays and/or phaserotation means are used to bring reflected ultrasound pulses arriving atdifferent transducer elements from the desired focal points intotemporal alignment and phase coherence. The time delays and phases usedto focus the ultrasound beam are specified assuming a constantpropagation velocity (nominally 1540 m/s in human soft tissue) in themedium through which ultrasound pulses propagate.

However, human soft tissue is not homogenous; it is composed of regionsof acoustically differing tissues, such as fat, muscle and blood, inwhich the local propagation velocity varies. The path dependent speed ofsound in tissue distorts the transmitted and reflected wavefrontspropagating through the tissues by introducing delay variations from thenominal. These delay variations degrade the quality of focus, thusreducing the spatial resolution and contrast resolution seen in theimage.

B. Patents and Literature

By way of example, the following United States patents and literature,all of which are incorporated by reference herein, discuss variousaspects of ultrasound imaging. The patents and literature include:

    ______________________________________                                        U.S.                                                                          Pat. No.:                                                                            Title:             Inventor(s):                                        ______________________________________                                        4,471,785                                                                            ULTRASONIC IMAGING David A. Wilson                                            SYSTEM WITH        James L. Buxton                                            CORRECTION FOR     Philip S. Green                                            VELOCITY IN-       Donald J. Burch                                            HOMOGENEITY AND    John Holzener                                              MULTIPATH INTER-   S. David Ramsey, Jr.                                       FERENCE USING AN                                                              ULTRASONIC IMAGING                                                            ARRAY                                                                  4,817,614                                                                            METHOD AND         Dietrich Hassler                                           APPARATUS FOR      Heinz Eschenbacher                                         ADAPTIVE FOCUSING  Wolfgang Haerer                                            IN A MEDICAL                                                                  ULTRASOUND IMAGING                                                            APPARATUS                                                              4,835,689                                                                            ADAPTIVE COHERENT  Matthew O'Donnell                                          ENERGY BEAM                                                                   FORMATION USING                                                               PHASE CONJUGATION                                                      4,852,577                                                                            HIGH SPEED ADAPTIVE                                                                              Stephen W. Smith                                           ULTRASONIC PHASED  Gregg E. Trahey                                            ARRAY IMAGING                                                                 SYSTEM                                                                 4,937,775                                                                            APPARATUS FOR THE  William E. Engeler                                         THE CROSS-         Matthew O'Donnell                                          CORRELATION OF                                                                A PAIR OF COMPLEX                                                             SAMPLED SIGNALS                                                        4,989,143                                                                            ADAPTIVE COHERENT  Matthew O'Donnell                                          ENERGY BEAM        Stephen W. Flax                                            FORMATION USING                                                               ITERATIVE PHASE                                                               CONJUGATION                                                            5,113,866                                                                            METHOD FOR         Dietrich Hassler                                           ULTRASOUND IMAGING Klaus Killig                                        5,172,343                                                                            ABERRATION         Matthew O'Donnell                                          CORRECTION USING                                                              BEAM DATA FROM                                                                A PHASED ARRAY                                                                ULTRASONIC SCANNER                                                     ______________________________________                                    

2. Literature

a. M. Hirama, et al., "Adaptive Ultrasonic Array Imaging System Throughan Inhomogeneous Layer," Journal of the Acoustical Society of America,Vol. 71, pp. 100-109, January 1982.

b. T. Yokota, et al., "Active Incoherent Ultrasonic Imaging Through anInhomogeneous Layer," Journal of the Acoustical Society of America, Vol.77, pp. 144-152, January 1985.

c. S. Flax, et al., "Phase Aberration Correction Using Signals FromPoint Reflectors and Diffuse Scatterers," IEEE Transactions onUltrasonics, Ferroelectrics, and Frequency Control, Vol. 35, pp.758-774, November 1988.

d. L. Nock, et al., "Phase Aberration Correction In Medical UltrasoundUsing Speckle Brightness As a Quality Factor," Journal of the AcousticalSociety of America, Vol. 85, pp. 1819-1833, May 1989.

e. M. O'Donnell, et al., "Correlation-Based Aberration Correction in thePresence of Inoperative Elements," IEEE Transactions on Ultrasonics,Ferroelectrics, and Frequency Control, Vol. 39, pp. 700-707, November1992.

All of the above disclose systems for determining aberration correctionsusing special adaptive modes nonconcurrent with imaging which may beused to correct nominal focusing delay and phase values during transmitbeamformation and/or receive beamformation for the defocusing effectscaused by aberrating regions. U.S. Pat. Nos. 4,471,785, 4,817,614 and4,852,577 show means to determine focusing corrections at a single depthand to apply the aberration correction values obtained during theadaptive mode to all focal points during the imaging modes.

However, because optimal aberration corrections vary as the focus isdynamically varied in depth during receive beamformation, correctionsdetermined at a single depth (or a few depths) do not optimally correctfocus at all depths. On the other hand, determining aberrationcorrections at many depths through direct measurement, as suggested byU.S. Pat. Nos. 4,835,689, 4,937,775, 4,989,143 and 5,172,343, mayrequire undesirable increases in 1) processing power, 2) computationtime (which may slow frame rate), 3) memory, and 4) the number ofnon-imaging scan lines (which further slows frame rate).

None of the related art is able to take aberration correction valuesobtained from one range, scan mode, geometry, and transmit frequency andapply them to alternative ranges, alternative scan modes, alternativescan geometries, and/or alternative transmit/receive frequencies. Forexample, if an imaging system were to acquire both color Doppler flowscan lines (color Doppler F-mode) using a steered linear scan geometryand gray scale image scan lines (B-mode) using a Vector® scan geometry,the related art systems would require separate aberration correctionvalues for each mode, geometry, and frequency and would not be able touse aberration correction values obtained from one mode, geometry, andfrequency to apply to the other modes, geometries, and frequencies.

Accordingly, it is desired to provide a method and apparatus fordetermining aberration correction values that can be applied for allfocal points for any scan mode, scan geometry or frequency withoutreducing frame rate or requiring special or separate acquisition modesapart from normal imaging modes.

SUMMARY OF THE INVENTION

An adaptive focusing ultrasound beamformer system operates inconjunction with an array which has a plurality of transducer elements,with each transducer element having an associated variable delay andgain. Ultrasound pulses are transmitted from the ultrasound arraythrough an aberrating region and focused at a specified transmit depthin the medium. Reflected ultrasound pulses, also distorted by passagethrough the aberrating region, are received on the same array. A firstset of aberration correction values (preferably the delay variations dueto the aberrations) for each transducer element at a specifiedmeasurement depth (preferably the transmit depth) are estimated. Theco-pending and above-identified patent application entitled: METHOD ANDAPPARATUS FOR REAL-TIME, CONCURRENT ADAPTIVE FOCUSING IN AN ULTRASOUNDBEAMFORMER IMAGING SYSTEM describes one aberration correction valuemeasurement approach.

The present invention provides a method for storing and retrievingaberration correction values measured and applied by an adaptivefocusing ultrasound imaging system. The method requires storage ofmeasured aberration correction values corresponding to each transducerelement or element subarray and a first set of measurement depths(preferably at the transmit focal point), and retrieval of a second setof aberration correction values by a geometric transform indexing ruleappropriate to each transducer element or element subarray and any depthlocation (preferably any transmit or dynamic receive focal point) in theimaged subject.

According to one aspect of the invention, measured correction values arestored during one imaging (scanning) mode, including, but not limitedto, B-mode (gray-scale imaging) or color Doppler F-mode (flow or colorDoppler imaging), and are retrieved and may be applied to any modes,including non-scanning modes such as spectral Doppler (D-mode) orM-mode.

According to a second aspect of the invention, measured aberrationcorrection values are stored during one scan geometry format, including,but not limited to, sector, Vector® linear curved linear, steeredlinear, steered curved linear and curved Vector®, and are retrieved andmay be applied to any other or the same scan geometry format.

According to another aspect of the invention, the apparatus preferablyincludes an index table coupled to an aberration correction value table(the values are preferably delay variations, although delay andamplitude variations are also possible values). Aberration correctionvalues obtained at a measurement depth are transformed to aberrationcorrection values at a requested image location. The transformationincludes the steps of generating index values in an index table used toretrieve a measured aberration correction value for use at any imagelocation. The measured aberration correction values are then placed inthe aberration correction value table. Index values from the index tableare selected responsive to requested element, scan line number, andlocation depth (or range) in order to retrieve the appropriateaberration correction value(s). The aberration correction value tableoutputs an aberration correction value responsive to the index valueoutput from the index table. Finally, a processor is coupled to theindex table for selecting a measurement scan line and measurement depth(range) when storing measured aberration correction values, and forretrieving a transformed aberration correction value for a requestedscan line and requested image depth (range).

According to yet another aspect of the invention, interpolation may beperformed between a first index value and a second index value.

Other aspects and advantages of the present invention can be seen uponreview of the figures, the detailed description, and the claims whichfollow.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1a and 1b conceptually depict the transmission and reception ofultrasound scan lines to and from body tissue.

FIGS. 2a-c illustrate an ultrasound imaging system, including therelationship of the geometric aberration transform to a transmitbeamformer, two receive beamformers, and an adaptive focus controlaccording to the present invention.

FIG. 3 illustrates a functional representation of a Geometric AberrationTransform (GAT™) apparatus showing data and control paths according tothe present invention.

FIG. 4 illustrates a distorted and a straight scan line passing throughan aberrating region to a focal point in a subject.

FIG. 5 illustrates a common path through an aberrating region resultingin the same delay correction value for an element at multiple focalpoints.

FIGS. 6a and 7 illustrate the use of measured aberration correctionvalues at a current focal depth according to the present invention.

FIGS. 6b and 6c illustrate a three-dimensional representation ofmeasured aberration correction values as a function of scan line numberand element number, and the use of a GAT™ index with the aberrationcorrection values, respectively, according to the present invention.

FIG. 8 illustrates use of a GAT™ function in different scan geometriesaccording to the present invention.

FIG. 9 illustrates use of a GAT™ function in different areas or zones ofan imaged subject according to the present invention.

FIG. 10 illustrates an example of calculating a GAT™ function for alinear transducer according to the present invention.

FIG. 11 illustrates an example of calculating a GAT™ functionnumerically according to the present invention.

FIG. 12 illustrates a measurement depth within the aberrating regionaccording to the present invention.

FIG. 13 illustrates a correction point within the aberrating regionaccording to the present invention.

FIG. 14 illustrates a GAT™ function referring to a non-existent scanline according to the present invention.

FIG. 15 illustrates a GAT™ function referring to a scan line numberoutside an active aperture according to the present invention.

FIG. 16 illustrates updating aberration correction values according tothe present invention.

FIG. 17 illustrates the interface of a GAT™ apparatus among an adaptivefocusing control system, digital receive beamformer system, beamformercentral control system and digital transmit beamformer system accordingto the present invention.

FIG. 18 illustrates a block diagram of a GAT™ apparatus architectureaccording to the present invention.

FIG. 19 illustrates a memory mapping of a GAT™ index table according tothe present invention.

FIG. 20 illustrates a memory mapping of a GAT™ high/low table accordingto the present invention.

FIG. 21 illustrates a memory mapping of a GAT™ delay table according tothe present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention represents a component of a medical ultrasoundimaging system for which additional patent applications, listed above,have been simultaneously filed in the United States Patent and TrademarkOffice. These applications are hereby incorporated by reference.

FIGS. 1a and 1b conceptually depict the transmission and reception ofultrasound scan lines to and from focal points such as r₁, r₂ and r₃ inbody tissue. FIGS. 2a-c illustrate an ultrasound imaging system with anadaptive focusing control system G-100. FIG. 3 depicts a block diagramof a GAT™ apparatus according to the present invention. FIGS. 4-18illustrate an implementation of a GAT™ apparatus and method. FIG. 17illustrates the interface between the digital receive beamformer systemR-100 and beamformer central control system C-104 in FIGS. 2b-c.Finally, FIGS. 19-21 illustrate a GAT™ indexing rule according to thepresent invention.

FIG. 3 depicts a functional data path and control path representation ofa GAT™ apparatus G-1. This apparatus may be used with any new or priorart adaptive focusing beamformer system that generates measurements ofaberration correction values, including the co-pending andabove-identified patent application entitled: METHOD AND APPARATUS FORREAL-TIME, CONCURRENT ADAPTIVE FOCUSING IN AN ULTRASOUND BEAMFORMERIMAGING SYSTEM. Measured aberration correction values from selectedmeasurement locations, preferably the transmit focus depth, are providedto GAT™ apparatus G-1 by an adaptive focus receive beamformer systemG-2, which are stored into GAT™ aberration correction value memory G-11after optional conversion of the measured aberration values. Forexample, the beamformer system G-2 may provide aberration correctionvalues as phase variations, which are converted by aberration correctionvalue conversion G-10 to equivalent delay values for storage to memoryG-11. A write address generation G-12 stores the aberration correctionvalues by measurement location. A location is uniquely identified byscan line number and depth (range) along the scan line. An adaptivefocus beamformer control system G-3, operating in conjunction with areceive beamformer and/or a transmit beamformer retrieves transformedaberration correction values for a requested image location (preferablya transmit or dynamic receive focal point) by looking up the correctionvalues using a GAT™ index function in the read address generation G-14.Optionally, aberration correction values stored in memory G-11 can beconverted as required by aberration correction value conversion G-13into values appropriate for adaptive focus beamformer control G-3, suchas conversion from delay correction values to phase correction values.

A. Definitions

1. Scan Lines

A scan line is a straight line through a space on which samples of animage are presumed to lie. A transmit scan line is such a line on whichan associated transmit beam is presumed to lie. A receive scan line issuch a line on which an associated receive beam is presumed to lie. Ascan line is spatially oriented by its origin along a transducer array,its azimuthal angle with respect to a line normal to the array face, andits elevation with respect to the normal line.

2. Subarrays

A subarray is any grouping of transducer elements, including the specialcase of a single transducer element. In the preferred embodiment of thisinvention, a subarray typically comprises four spatially adjacentelements.

3. Measured Aberration Correction

Value(s)

Measured aberration correction values are values generated by anadaptive focusing beamformer system, that represent metrics ofvariations at a measurement depth for a given transducer element in theparameters that affect transmit focusing, receive focusing, or both, dueto an aberrating region in the propagating medium. In the preferredembodiment, the values are delay variations (possibly converted, forexample, from element-to-element phase differences), although othervalues may be used, such as phase variations or amplitude variations.Measured correction values and measured values are used interchangeablyherein for measured aberration correction values.

4. Transformed Aberration Correction Values

Transformed aberration correction values are output values from a GAT™apparatus produced by a request from an adaptive focusing beamformersystem and represent metrics of variations at a requested image locationfor a given transducer element in the parameters that affect transmitfocusing, receive focusing, or both, due to an aberrating region in thepropagation medium. In the preferred embodiment, the values are delayfocusing corrections for both transmit and receive beamforming, althoughother values are possible, such as both delay and amplitude focusingcorrections. Transformed correction values and transformed values areused interchangeably for transformed aberration correction values.

5. Correction Profile

A correction profile is a sequence of aberration correction values as afunction of transducer element position.

6. Location

A location is a point position in the image uniquely determined by scanline number and depth (range) along that scan line from the scan lineorigin.

7. Current Location

The location of the current scan line and current range in a scangeometry format that evolves with time.

8. Scan Line Number

The scan line number is a single index that uniquely corresponds to thethree spatial location attributes (origin, azimuthal angle, andelevational angle) of a scan line, thereby simplifying the indexingschemes of the GAT™ tables.

9. GAT™ Index Value

The GAT™ index value is a scan line number output by the GAT™ indexfunction and used to look up the transformed aberration correctionvalue.

10. Correction Location

The correction location, or correction point, is a point for which atransformed aberration correction value is requested, uniquelydetermined by a correction scan line number and a correction depth. Inthe apparatus section below, correction location, correction scan line,and correction depth are alternatively referred to as current focalpoint, current scan line, and current depth, respectively.

B. Overview of the Preferred Beamformer System Architecture:

1. Ultrasound Signal Description:

With respect to the present invention, ultrasound imaging isaccomplished by firing (transmitting) into body tissue or other objectsto be imaged a scan sequence of focused ultrasonic beams centered alongstraight lines in space called transmit scan lines (FIG. 1a). Thetransmit scan lines are generated by a transmit beamformer and anultrasound transducer array. The transmit scan lines are spaced toproduce a planar linear, planar sector or other display of the tissuevia a pre-defined firing or scanning pattern. Focused to some defineddepth in the tissue, the ultrasonic transmit continuous-wave (CW) orpulse-wave (PW) signal, propagating at an assumed constant propagationvelocity of nominally c=1540 m/sec through the tissue, interacts withthe tissue and reflects a small portion of the signal back to theultrasound transducer array that initiated the ultrasound signal. Theround trip delay time is shortest for those targets closest to theultrasound transducer array, and longest for those targets farthest fromthe transducer array. With the application of appropriate time delays,the receive beamformer (FIG. 1b) can dynamically focus receive beamsalong straight lines in space called receive scan lines commencing, forexample, with the shallowest range (depth) of interest and evolvingtoward the deepest range of interest.

FIGS. 1a and 1b depict representations of transmit and receive scanlines (solid) and straight-line signal propagation paths from individualelements (dashed), respectively. In FIG. 1a, the transmit beamformer isgenerally identified by T-50 with the transducer array T-52 containing amultiplicity of individual transducer elements T-54 organized as alinear phased array in this particular embodiment. As is known in theart, there are a great variety of transducer array configurationsavailable for use with ultrasound transmit and receive beamformersystems. As can be seen in FIG. 1a, the transmit beamformer T-50 sendsappropriately time-delayed electrical signals to the individualtransducer elements T-54. These transducer elements T-54 then in turnconvert electrical signals into acoustic waves that propagate into thebody tissue T-56. By applying different time delays to the excitationsignals sent to the individual transducer elements T-54, transmit scanlines T-60 and T-62, having respective foci r₁ and r₂, can beestablished. It is to be understood that each of these transmit scanlines is representative of a center line of a different transmit beamwhich is steered and focused into the body to be imaged.

The transmit beamformer T-50 can generate simultaneous multiple beamsalong different scan lines, or different focal depths along the samescan line (compound focus). Further, the multiple transmit beams caneach scan the entire image format or be transmitted such that each ofthe multiple beams only scans a specified section of the image format.

FIG. 1b depicts a digital receive beamformer R-58 which is alsoconnected to the transducer array T-52. Also depicted in FIG. 1b arereceive scan lines R-64, R-66 corresponding to a dynamically focusedfirst receive beam and a dynamically focused second receive beam,respectively. The beams are sampled in range at a plurality of focaldepths (r₁, r₂, r₃) along each scan line. In the digital receive signalpath of the present invention, transducer array signals can beselectively separated into data representative of multiple individualbeams.

Each scan line of a transmit or receive scan pattern can beparameterized by the origin on the transducer array, the scan lineorientation (angle θ) and the focus depth or range (r). The ultrasoundimaging system of the present invention stores a pre-computed sparsedata set of focusing time delay and aperture apodization values indexedby these parameters (based on geometric considerations as is known inthe art) and expands the values by real-time computational means tocontrol the transmit and receive beamformation systems that produce thedesired scan lines.

2. Beamformer System:

FIGS. 2a, 2b, 2c depict an overall block diagram of a medical ultrasoundimaging system R-20. Ultrasound system R-20 includes a beamformer systemR-22, one or more transducers T-112, a display processing system R-26with a display R-28 and an ultrasound imaging system control R-40.

In FIGS. 2a, 2b, or 2c, the beamformer system R-22 includes inventiveand novel (1) digital transmit beamformer system T-102, (2) digitalreceive beamformer system R-100, (3) beamformer central control systemC-104, (4) adaptive focusing control system G-100 and (5) Dopplerreceive beamformer system A-400. These systems are depicted as highlevel, functional block diagrams. The blocks are abstracted from theactual implementation of a preferred embodiment in order to betterillustrate the signal processing functions performed.

As indicated in FIG. 2a, beamformer system R-22 provides two sources ofdigital beam data to the display processing system R-26: (1) Dopplerreceive beamformer single-beam complex in-phase/quadrature datarepresenting coherent temporal sampling of the beam (CW case) orcoherent temporal sampling at one range location along the beam (PWcase), and (2) digital receive beamformer multi-beam complexin-phase/quadrature data representing coherent sampling in range alongeach receive scan line. Beamformer system R-22 can be operated toprovide a sequence of scan lines and associated samples as above toprovide data for a variety of display modes. By way of example, possibledisplay modes and their associated processors include (1) brightnessimage and motion processor R-30 for B-mode (gray-scale imaging) andM-mode (motion display), (2) color Doppler image processor R-32 forF-mode (flow imaging), and (3) spectral Doppler processor R-34 forD-mode. Additional display modes can be created from the two complexdata sources of R-22, as will be obvious to those skilled in the art.

Ultrasound system R-20 also includes a transmit demultiplexer T-106 forrouting the output waveforms from the transmitters T-103 to thetransducer elements T-114, a receive multiplexer R-108 for routing theinput waveforms from the transducer elements T-114 to the receiversR-101, one or more transducer connectors T-110 and transducer arraysT-112. Many types of transducer arrays can be used with the presentsystem.

Ultrasound system R-20 also includes an ultrasound imaging systemcontrol R-40, archival memory R-38 for storing scan parameters and scandata, and operator interface R-36.

As used herein, the term ultrasonic refers to frequencies above therange of human hearing. However, the transducer arrays T-112 areoptimized for frequencies typically within the range of 2-10 MHz.

The transducer array T-112 is interchangeable with a variety ofdifferent kinds of transducer arrays, including but not limited tolinear, curved, curvi-linear and annular transducer arrays. A variety oftransducer array shapes and frequencies are desirable in order tosatisfy the requirements of a variety of different clinical settings.However, the transducer arrays T-112 are typically optimized forfrequencies within the above specified range of 2-10 MHz. The medicalultrasound system R-20 performs the three major functions of driving theultrasonic transducer array of elements T-114 to transmit focusedultrasound energy, receiving and focusing back-scattered ultrasoundenergy impinging on the transducer array T-114, and controlling thetransmit and receive functions to scan a field of view in scan formatsincluding (but not limited to) linear, sector or Vector® format.

In FIGS. 2a, 2b, 2c, the control signals are communicated over the lightlead lines while the signal paths are depicted with heavy lead lines.

3. Digital Transmit Beamformer System:

The digital transmit beamformer T-102 (FIG. 2c) is the subject of theabove-identified application entitled: METHOD AND APPARATUS FOR TRANSMITBEAMFORMER SYSTEM which has been incorporated herein by reference. It isto be understood that in a preferred embodiment, the digital transmitbeamformer T-102 is comprised of a plurality of digital multi-channeltransmitters T-103, one digital multi-channel transmitters for one ormore of the individual transducer elements T-114. The transmitters aremulti-channel in that each transmitter can process, in a preferredembodiment, up to four independent beams. Thus, for example, 128multi-channel transmitters have 512 channels. In other preferredembodiments, more than four independent beams can be achieved.Processing more than four beams per processor is within the scope of theinvention.

In a preferred embodiment, each of the digital multi-channeltransmitters T-103 produces as its output in response to an excitationevent the superposition of up to four pulses, each pulse correspondingto a beam. Each pulse has a precisely programmed waveform, whoseamplitude is apodized appropriately relative to the other transmittersand/or channels, and delayed by a precisely defined time delay relativeto a common start-of-transmit (SOT) signal. Transmitters T-103 are alsocapable of producing CW.

Each digital multi-channel transmitter T-103 conceptually comprises amultiple beam transmit filter T-115 which provides an output to acomplex modulator T-117. The output from complex modulator T-117 iscommunicated to a delay/filter block T-119, and therefrom is provided toa digital-to-analog converter (DAC) T-121. The output of the DAC T-121is amplified by an amplifier T-123. The multiple beam transmit filterT-115, the complex modulator T-117 and the delay/filter block T-119comprise a digital multi-channel transmit processor T-104.

The transmit filter T-115 can be programmed to provide any arbitraryreal or complex waveform responsive to a start-of-transmit (SOT) signal.The transmit filter T-115 is implemented with a memory which stores realor complex samples of any desired and arbitrary pulse waveform, and ameans of reading the samples out sequentially in response to thestart-of-transmit (SOT) signal delayed by a component of the focusingdelay. In a preferred embodiment, the memory of T-115 is programmed tostore baseband representations of real or complex pulse envelopes.

Block T-115, although primarily a memory, is referred to herein as atransmit filter, as the output of block T-115 can be thought of as thetime response of a filter to an impulse. The complex modulator T-117upconverts the envelope to the transmit frequency and providesappropriate focusing phase and aperture apodization.

Delay/filter block T-119 conceptually provides any remaining focusingdelay component and a final shaping filter. The digital-to-analogconverter (DAC) T-121 converts the transmit waveform samples to ananalog signal. The transmit amplifier T-123 sets the transmit powerlevel and generates the high-voltage signal which is routed by thetransmit demultiplexer T-106 to a selected transducer element T-114.

Associated with each multi-channel transmit processor T-104 is a localor secondary processor control C-125 which provides control values andparameters, such as apodization and delay values, to the functionalblocks of multi-channel transmit processor T-104. Each local orsecondary channel control C-125 is in turn controlled by the central orprimary control system C-104.

4. Digital Receive Beamformer System:

The digital receive beamformer R-100 (FIG. 2b) is the subject of theabove-identified application entitled: METHOD AND APPARATUS FOR RECEIVEBEAMFORMER SYSTEM which has been incorporated herein by reference.

The signals from the individual transducer elements T-114 representreturn echoes or return signals which are reflected from the objectbeing imaged. These signals are communicated through the transducerconnectors T-110 to the receive multiplexer R-108. Through multiplexerR-108, each transducer element T-114 is connected separately to one ofthe plurality of digital multi-channel receivers R-101 which takentogether and along with the baseband multi-beam processor R-125 comprisethe digital receive beamformer R-100 of the invention. The receivers aremulti-channel in that each receiver can process, in a preferredembodiment, up to four independent beams. Processing more than fourbeams per processor is within the scope of the invention.

Each digital multi-channel receiver R-101 can, in a preferredembodiment, comprise the following elements which are represented by thehigh level function block diagram in FIG. 2b. These elements include adynamic low-noise and variable time-gain amplifier R-116, ananalog-to-digital converter (ADC) R-118, and a digital multi-channelreceive processor R-120. The digital multi-channel receive processorR-120 conceptually includes a filter/delay unit R-122 and a complexdemodulator R-124. The filter/delay unit R-122 provides for filteringand coarse focusing time delay. The complex demodulator R-124 providesfor fine focusing delay in the form of a phase rotation and apodization(scaling or weighting), as well as signal demodulation to or nearbaseband. The exact functioning and composition of each of these blockswill be more fully described hereinbelow with respect to the remainingfigures.

The digital multi-channel receivers R-101 communicate with basebandmulti-beam processor R-125 where the signal samples of each beam fromeach receive processor are summed by summer R-126, and the summationprovided to baseband filter/phase aligner R-127. The basebandfilter/phase aligner R-127 provides for filtering, andreceive-scan-line-to-receive-scan-line or beam-to-beam phase alignmentas discussed in the above-referenced and incorporated patentapplications entitled: METHOD AND APPARATUS FOR COHERENT IMAGEFORMATION, METHOD AND APPARATUS FOR ADJUSTABLE FREQUENCY SCANNING INULTRASOUND IMAGING, and METHOD AND APPARATUS FOR RECEIVE BEAMFORMERSYSTEM.

A local or secondary control C-210 is associated with each digitalmulti,channel receiver R-101. Local multi-channel processor controlC-210 is controlled by central or primary control C-104 and providestiming, control and parameter values to each said receiver R-101. Theparameter values include time delay values and apodization values.

The digital receive beamformer system R-100 additionally includes abaseband processor control (or phase aligner processor control) C-270which controls the operation of baseband filter/phase aligner R-127 andthe summing gain of summer R-126. Baseband processor control C-270 iscontrolled by central control C-104.

5. Doppler Receive Beamformer System:

The Doppler receive beamformer system A-400 for D-mode acquisitionincludes analog receivers A-402, each of which receives echo signalsfrom a respective one or more transducers T-114. Each of the Dopplerreceivers A-402 includes a demodulator/range gate A-404 whichdemodulates the received signal and gates it (PW mode only) to selectthe echo from a narrow range. The analog outputs of the Dopplerreceivers A-402 are communicated to a Doppler preprocessor A-406. Inpreprocessor A-406, the analog signals are summed by summer A-408 andthen integrated, filtered, and sampled by analog processor A-410.Preprocessor A-406 then digitizes the sampled analog signal in ananalog-to-digital converter (ADC) A-412. The digitized signal iscommunicated to the display processing system R-26. The Doppler receivebeamformer system is the subject of a co-pending patent applicationentitled: METHOD AND APPARATUS FOR DOPPLER RECEIVE BEAMFORMER SYSTEMwhich has been incorporated herein by reference.

Associated with all Doppler receivers A-402 is a single local orsecondary Doppler beamformer control C-127. Doppler beamformer controlC-127 is controlled by central or primary control system C-104 andprovides control and focusing parameter values to the Doppler receivebeamformer system A-400.

As pointed out in the above patent application describing the Dopplerreceive beamformer system A-400, the present beamformer system R-22advantageously combines a digital receive beamformation system R-100 anda Doppler receive beamformation system A-400 in a manner which uses thesame digital transmit beamformation system T-102 and the same transducerarray and allows the digital receive beamformation system R-100 to beoptimized for imaging modes such as B- and F- mode, and therefore hashigh spatial resolution, while the accompanying Doppler receivebeamformation system has a wide dynamic range and is optimized for usein acquiring D-mode signals.

6. Beamformer Central Control System:

The beamformer central control system C-104 of the present inventioncontrols the operation of the digital transmit beamformer system T-102,the digital receive beamformer system R-100, the Doppler receivebeamformer system A-400, and the adaptive focusing control system G-100.The beamformer control is more fully discussed in the above referencedand incorporated patent application entitled: METHOD AND APPARATUS FORFOCUS CONTROL OF TRANSMIT AND RECEIVE BEAMFORMER SYSTEMS.

The main control functions of the central control system C-104 aredepicted in FIG. 2c. The control functions are implemented with fourcomponents. The acquisition control C-130 communicates with the rest ofthe system including the ultrasound system control R-40 and provideshigh level control and downloading of scanning parameters. The focusingcontrol C-132 computes in real time the dynamic delay and apodizationdigital values required for transmit and receive beamformation, whichincludes pre-computed and expanded ideal values plus any estimatedcorrection values provided by adaptive focusing control system G-100.The front end control C-134 sets the switches for the demultiplexerT-106 and the multiplexer R-108, interfaces with the transducerconnectors T-110, and sets the gain and bias levels of all transmitteramplifiers T-123 and all receive amplifiers R-116. The timing controlC-136 provides all the digital clocks required by the digital circuits.This includes the sampling clocks for all the transmitter DACs T-121 andreceiver ADCs R-118.

In a preferred embodiment central control C-104 expands sparse tables offocusing time delay and aperture apodization values based onpre-computed and stored data, through such techniques as interpolationand extrapolation. The expanded delay and apodization values arecommunicated to the local processor controls, where the delay andapodization data expansion in range is completed toper-transducer-element, per-sample, per-beam values.

7. Adaptive Focusing Control System:

Adaptive focusing control system G-100 provides for real time concurrentadaptive focusing. Adaptive focusing control system G-100 is comprisedof an adaptive focus processor G-505 which provides focus correctiondelay values to the focus control C-132 of the central control C-104.Adaptive focus processor G-505 operates on output produced by aberrationvalue estimators G-502 from data gathered from the subarray summersR-126 of the digital receive beamformer system R-100. Accordingly,aberration correction values, preferably aberration delay and amplitudevalues, are adaptively measured for each receive scan line or for asubset of receive scan lines in range regions corresponding to transmitfocal depths by the adaptive focusing control subsystem G-100 shown inFIG. 2c. Adaptive focusing control system G-100 is more fully describedin the above-referenced and incorporated co-pending patent applicationentitled: METHOD AND APPARATUS FOR REAL-TIME, CONCURRENT ADAPTIVEFOCUSING IN AN ULTRASOUND BEAMFORMER IMAGING SYSTEM.

It is to be understood that in addition to the adaptive focusing controlsystem which adjusts focus delays, that a number of adaptive controlsystems are contemplated. These systems, by way of example, include (1)adaptive contrast enhancement control system for adjusting focus delaysand aperture apodizations, (2) adaptive interference cancellationcontrol for adjusting focus delays and phases, aperture apodizations,and (3) adaptive target enhancement control for adjusting focus delaysand phase, aperture apodizations, imaging transmit and receivefrequencies and baseband waveform shaping.

Another aspect of adaptive focusing which can be included in thepreferred embodiment of the adaptive focusing control system G-100 is ageometric aberration transform (GAT™) device G-508/509 which can provideaberration correction delay values to the adaptive focus processor G-505for scan lines and scan line depth locations for which measuredaberration values were not collected by aberration value estimatorsG-502. More specifically, measured aberration correction values arewritten to a delay table in GAT™ G-508/509. GAT™ G-508/509 retrievesvalues from the delay table according to GAT™ look-up rules to formfocusing delay correction profiles across the aperture valid for depths,scan geometries, and acquisition modes other than the depth, scangeometry, and mode at which aberration correction values were measured.The geometric aberration transform device G-508/509 is more fullydescribed below.

C. Method

This section refers to the storage and retrieval of correction valuesfor elements of a transducer array. The invention and the text belowapply equally to subarrays, groupings of a small number of adjacentelements; "elements" are used in the text to clarify the conceptsdisclosed.

The text also refers to "correction values." In the preferredembodiment, and in the text below, these are delay correction values(stored in a "delay table"). As described below, the invention appliesequally to a broader class of measured correction values, includingamplitude correction values and phase correction values.

1. Aberration Correction Values in Scan Regions Beyond AberratingRegions

In a phased-array ultrasound system, ideal focusing delays T_(ideal) foreach element are calculated to compensate for the variation inpropagation time from element E at position E to a desired focal pointP, illustrated in FIG. 4: ##EQU1## where c_(O) is the nominal speed ofsound through a homogeneous medium and |P-E| represents the distancebetween P and E.

Equation (1) can also be written as a simple path integral along astraight dashed line G-202 from element position E to focal point P:##EQU2## These desired focusing delays T_(ideal) can be implemented inan ultrasound imaging system using time delay, phasing, or, in thepreferred embodiment, a combination of the two.

In a subject having an aberrating region G-201, the propagation delayT_(inhom) (and hence the desired focusing delay) generally depends onthe speed of sound through an aberrating region G-201. T_(inhom) dependson the propagation time along a distorted (refracted) path representedby curved line G-200 from element position E to focal point P. Inintegral form, ##EQU3## where n(x,y) is the index of refraction definedby n(x,y)≐|c_(O) /c(x,y)|, and c(x,y) is the spatially varying localspeed of sound at a point (x,y) within the propagating medium (whichincludes both the aberrating region and the non-aberrating region).

For the case of relatively small variations in the speed of sound, theintegral in equation (3) can be approximated by an integral over astraight line G-202 from element position E to focal point P, or##EQU4## The focusing delay error T_(ab), the aberration correctionvalue due to aberrating region G-201, can be defined as: ##EQU5## Inother words, the focusing delay error T_(ab) for element E and focalpoint P is determined by integrating over the variation (from nominal)in the index of refraction on a straight line G-202 from elementposition E to focal point P.

The aberrating region G-201 (the region for which the speed of sounddiffers from the nominal value c_(O)) is assumed to have limitedthickness. Because outside this region n(x,y)=1, the aberrationcorrection value T_(ab) defined in equation (5) has the same value forany set of focal points outside the aberrating region lying on astraight line from the given element position E. FIG. 5 illustrates thisgraphically. Because focal points P₀ through P₅ lie on a straight linepassing through element position E, and lie outside the aberratingregion G-201 delineated by the dotted line G-203, the propagation pathsfrom element E to each focal point P₀ to P₅ pass through the sameportion of the aberrating region G-201. Thus the aberration correctionvalue T_(ab) corresponding to element E for each focal point P₀ to P₅ isidentical.

FIG. 6a shows how the principle above can be used to generate aberrationcorrection profiles across the aperture at depths other than thosedepths at which correction values are measured. FIG. 6a shows aneight-element linear array E₀ -E₇, which has generated fifteen parallelscan lines L_(m0) -L_(m14), each normal to the array, in order to obtainaberration correction values (only scan lines L_(m12), L_(m13) andL_(m14) are shown in FIG. 6a). Aberration correction profiles aremeasured at fifteen measurement focal points P_(m0) -P_(m14), one oneach scan line at a measurement depth D_(m) (measured along the scanlines). For example, the measured correction values for focal pointP_(m0) and elements E₁ -E₅ represent delay variations along propagationpaths shown by dashed lines G-205.

The aberration correction values T_(meas) (E_(i),L_(mj),D_(m)),corresponding to i^(th) element E_(i) and j^(th) measurement scan linenumber L_(mj) at the measurement depth D_(m), are stored in a tableG-400, represented in FIGS. 6b-c. Suppose it is now desirable to correctfocus at a focal point P_(c) at depth D_(c) in FIG. 6a. Element positionE₀ and measurement focal point P_(m3) are collinear with focal pointP_(c). Therefore, the aberration correction value applicable to elementE₀ at focal point P_(c) is the same as the correction value for elementE₀ at measurement point P_(m3).

This relationship can be written:

    T.sub.corr (E.sub.0,P.sub.c)=T.sub.meas (E.sub.0,P.sub.m3) (6)

where T_(corr) (E_(i),P_(c)) is the correction value for focal pointP_(c) and element E_(i), and T_(meas) (E_(i),P_(mj)) is the measuredcorrection value for element E_(i) and measurement focal point P_(mj).Likewise, measurement focal point P_(m4) is collinear with elementposition E₁ and the correction point P_(c), so

    T.sub.corr (E.sub.1,P.sub.c)=T.sub.meas (E.sub.1,P.sub.m4).(7)

Continuing likewise through all the elements, we can extract from themeasured aberration correction values a complete correction profile (thefocusing delay error to be corrected, as a function of element number)valid at the desired focal point P_(c) :

    T.sub.corr (E.sub.2,P.sub.c)=T.sub.meas (E.sub.2,P.sub.m5)

    T.sub.corr (E.sub.3,P.sub.c)=T.sub.meas (E.sub.3,P.sub.m6)

    T.sub.corr (E.sub.4,P.sub.c)=T.sub.meas (E.sub.4,P.sub.m7) (8)

    T.sub.corr (E.sub.5,P.sub.c)=T.sub.meas (E.sub.5,P.sub.m8)

    T.sub.corr (E.sub.6,P.sub.c)=T.sub.meas (E.sub.6,P.sub.m9)

    T.sub.corr (E.sub.7,P.sub.c)=T.sub.meas (E.sub.7,P.sub.m10)

2. GAT™ Function

In the example given above, we can restate equations (6), (7), and (8)as:

    T.sub.corr (E.sub.i,P.sub.c)=T.sub.meas (E.sub.i,G(E.sub.i,P.sub.c,D.sub.m))(9)

Equivalently, we can replace the vector notation with a coordinatenotation, substituting L_(c) and D_(c) for P_(c), and G(.) and D_(m) forG(.):

    T.sub.corr (E.sub.i ;L.sub.c,D.sub.c)=T.sub.meas (E.sub.i ;G(E.sub.i ;L.sub.c,D.sub.c ;D.sub.m).                               (10)

where L_(c) is the scan line number and D_(c) the depth of correctionfocal point P_(c).

G(.) in equation (10) is the GAT™ index function which relates therequired correction at an element E_(i), current line L_(c) and currentdepth D_(c) to an appropriate measured correction valve at a depth D_(m)along a scan line L_(mj) =G(E_(i) ;L_(c),D_(c) ;D_(m)). The presentinvention determines values of the GAT™ index function and implementsthe mapping defined in equation (10). FIG. 6c illustrates the use ofGAT™ index function G-401 with a GAT™ aberration correction value tableG-400. Aberration correction value table G-400 contains measured delaycorrection values at measurement depth D_(m) corresponding to elementsE_(i) and measurement scan lines L_(mj). For element E₆, correction scanline L_(c), measurement depth D_(m), and correction depth D_(c), GAT™index function G-401 produces a GAT™ index value G(E₆;L_(c),D_(c),D_(m)) at G-404 that corresponds to the appropriate scanline number L_(m) for which the measured correction value T_(meas)(E₆,L_(m),D_(m)) may be found in aberration correction value tableG-400. In this example,

    L.sub.m =G(E.sub.6 ;L.sub.c ;D.sub.c ;D.sub.m)             (11)

The aberration correction value T_(meas) (E₆,L_(m),D_(m)) is used tocorrect the focus at element E₆, scan line L_(c), and correction depthD_(c).

FIG. 6b shows measured aberration correction values for a measurementdepth D_(m) as in FIG. 6a, plotted as a function of measurement scanline number L_(mj) and element number E_(i). The vertical values on eachcurve G-15 represent the measured correction values for each measurementscan line L_(m2), L_(m3), etc., plotted as a function of element numberE₀, E₁, etc. Curve G-20 represents the locus of the GAT index functionG(E_(i) ;L_(c) ;D_(c) ;D_(m)) for correction point P_(c) (correctionline L_(c) and correction depth D_(c)) as in FIG. 6a, plotted vs.element number in the "element number"-"measurement scan line number"plane of the drawing. Transformed correction values G-30 are obtainedfor each element E_(i) according to the GAT index value for thatelement, as described above. Transformed correction profile G-40 isformed by taking transformed correction values G-30 for each elementE_(i), and is plotted as transformed aberration correction value vs.element number on graph G-50. Transformed correction profile G-40 isessentially a "slice" through the measured aberration correction values,projected onto the "delay error"-"element number" plane.

These transformed aberration correction values extracted frommeasurements made in one measurement mode may then be used in the sameor other modes, such as B-mode (gray-scale imaging mode), color DopplerF-mode (flow or color Doppler imaging mode), M-mode (motion imagingmode), and D-mode (spectral Doppler mode), or in interleaved oralternating combinations of these modes.

For any correction point P_(c), regardless of the scan geometry withinwhich that point is defined, values of the GAT™ index function G(.) canbe determined as a geometry problem. This allows aberration correctionsmeasured in one scan geometry to be used to correct the focus in adifferent scan geometry. FIG. 8 illustrates one such possibility: B-modemeasurement scan lines L_(mj) (shown as dashed lines) are generated in aVector® scan geometry, and aberration measurements are made at pointsP_(m0) -P_(m7) located along these scan lines at the measurement depthD_(m) from the face of the transducer array. Interleaved color DopplerF-mode scan lines L_(ck) (shown as dotted lines) are generated in asteered linear scan geometry, and the resulting color Doppler F-modeimage is overlaid onto the B-mode image, with focus corrected using datameasured during the B-mode, Vector® scan geometry. To correct the focusfor correction point P_(c) (in the steered linear scan geometry format)and element E₁ at position E₁, the measured correction value for pointP_(m6) (collinear with E₁ and P_(c)) in the Vector® scan geometry formatis used. Equation (10) applies to this case, but here D_(c) and L_(c)are depth and scan line number in the correction scan geometry (e.g.,steered linear), and D_(m) and the GAT™ index itself are depth and scanline number in the measurement scan geometry (Vector®). The GAT™ indextherefore transforms aberration correction values from one scan geometryto another as well as from one depth to another. Note that the form ofthe GAT™ index (the value associated with a particular element,correction scan line and depth) changes, but the geometric relationshipused to determine the GAT™ index for a particular correction point inspace remains the same.

The primary approach to the present invention relies on sampled valuesof a GAT™ index function being stored in a table. Calculation of theseindex values is a geometric problem that may be solved to arbitraryaccuracy by one skilled in the art using numerical methods, and isdescribed further in section V.B.3., Calculating the GAT™ Function. TheGAT™ index is calculated for a subset of possible measurement scanlines, called reference scan lines L_(Rj) for element E_(i) at acorrection depth D_(c) and measurement depth D_(m), and stored in atable. When correcting the focus for one of these reference scan lines,the GAT™ index value is read directly from the table. When correctingthe focus for some scan line L_(c) other than a reference scan line, theGAT™ index is generated by interpolation. For example, if scan lineL_(c) lies between reference scan lines L_(R1) and L_(R2), the GAT™index value for scan line L_(c), G(E;L_(c),D_(c) ;D_(m)), is obtained byinterpolation between the GAT™ index values for reference scan linesL_(R1) and L_(R2) : ##EQU6## where ##EQU7##

This approach has the advantage of separating the generation of valuesof an ideal GAT index function G(.) from the real-time processing of theGAT index function. However, those skilled in the art could devise othermethods to generate values of a GAT™ index function. For example, itcould be calculated in real time, using either an exact analytic formulaor an approximation, such as a power series expansion.

In the example illustrated in FIG. 6a, propagation paths to correctionpoint P_(c) from each element E₀ -E₇ always pass directly through one ofthe aberration measurement focal points P_(m0) -P_(m14). This is usuallynot the case.

As can be seen from FIG. 7, when generating correction values forelement E₅, measured aberration correction values for measurement pointsP_(m5) and P_(m6) may be used respectively as transformed aberrationcorrection values for correction points P_(c5) and P_(c6). However, whencorrecting at some correction point P_(c7) between P_(c5) and P_(c6), anaberration correction value for a point P_(m7) between P_(m5) and P_(m6)is required. Because no aberration correction value was measured forthis point, the aberration correction value T_(corr) (P_(c7)) to beapplied to correction point P_(c7) is determined by taking either themeasured correction value T_(meas) (P_(m5)), measured at point P_(m5),or T_(meas) (P_(m6)) measured at point P_(m6), whichever is closer toP_(m7).

In another embodiment, the aberration correction value T_(corr) (P_(c7))may be calculated by interpolating between T_(meas) (P_(m5)) andT_(meas) (P_(m6)):

    T.sub.corr (P.sub.c7)=T.sub.meas (P.sub.m7)=α.sub.L T.sub.meas (P.sub.m5)+(1-α.sub.L)T.sub.meas (P.sub.m6)         (14)

where α_(L) is a weighting based on scan line number, steering angle, orsome other scan line dependent parameter. For example, to interpolate inscan line number, ##EQU8## where L_(m5), L_(m6) and L_(m7) are themeasurement scan lines on which, respectively, P_(m5), P_(m6) and P_(m7)are located.

In addition, the above principles may be extended to measurement schemesthat obtain measured aberration correction values at several depths,creating zones in an imaged subject as illustrated in FIG. 9. Forexample, a first zone may be defined as the space between the transducerelements and some boundary depth D_(b), containing a measurement depthD_(m1). A second zone may be defined deeper than the boundary depthD_(b), containing a second measurement depth D_(m2). Two sets of GAT™indices are then defined, one transforming to measurement range D_(m1)and one transforming to measurement range D_(m2). For all focal points(such as P_(c1)) within the first zone (shallower than D_(b)),correction is performed using the first GAT™ index set and measurementdepth D_(m1), while focal points (such as P_(c2)) deeper than D_(b), arecorrected using the second GAT™ index set and measurement depth D_(m2).

The discussion above has centered on the use of the invention to extracttransformed delay correction values from measured delay aberrationvalues. In alternative embodiments, the invention may be appliedusefully to any aberration correction values measured for elements andmeasurement points, where 1) the underlying values depend only on thestraight line path from each element to each correction point through anaberrating medium; 2) the measured values vary smoothly with elementnumber; and 3) the measured values vary smoothly with scan line.

3. Calculating The GAT™ Index Function

Calculating the GAT™ index function is a geometric problem that may besolved by a number of methods. It may be calculated while scanning,calculated when scanning is initiated, or pre-calculated and stored in alarge table. For some scan geometries (including, but not limited tolinear, steered linear, curved linear, and sector), exact analyticexpressions may be used; for others, approximations may be required. Forall formats, the GAT™ function can be calculated by one of ordinaryskill in the art to arbitrary accuracy by numerical methods, or to lessaccuracy by more computationally efficient methods.

To illustrate the principles of GAT™ index function calculations, FIG.10 shows an example of the exact calculation of a GAT™ index for aplanar transducer operating in a linear scan geometry (which producesequi-spaced parallel scan lines). A correction is desired for elementE_(n) with coordinates (X_(e),O) focused at correction point P_(c) withcoordinates (X_(c),D_(c)) on scan line L_(c). The lateral coordinateX_(m) of the measurement point P_(m) is given by: ##EQU9## This may berewritten ##EQU10## where ΔX_(m) =X_(m) -X_(c) is the lateral distancefrom measurement point P_(m) to correction scan line L_(c) and ΔX_(e)=X_(e) -X_(c) is the lateral distance from element E to correction scanline L_(c).

To calculate a usable GAT™ index, all that remains is to translate thelateral coordinate X_(m) of the measurement into measurement scan linenumber L_(m). If the spacing between scan lines is S₁ such that X_(m)and X_(c) are related to measurement scan line number L_(m) andcorrection scan line number L_(c)

    X.sub.m =L.sub.m.S.sub.1                                   (18)

    X.sub.c =L.sub.c.S.sub.1                                   (19)

then equation (16) above may be rewritten ##EQU11## Thus equation (20)is the exact GAT™ index function for the scan geometry depicted by FIG.10.

FIG. 11 illustrates an alternative example of a way to calculate GAT™index values by numerical approximation rather than exact analyticevaluation:

1) Start with the element (E₇ in the FIG. 11) closest to the origin ofthe correction scan line L_(c). For this element, the GAT™ indexfunction will return approximately the correction scan line number.

2) Using this estimate as a starting point, a more accurate estimate ofthe GAT™ index may be formed as described below. This step may beiterated as necessary, using each new estimate as a basis to makefurther refinements.

3) Proceeding to an adjacent element E₆, this new estimate of the GAT™index for element E₇ is used as a first estimate to calculate the indexfor element E₆. Each subsequent estimate of the GAT™ index is used as astarting point to approximately calculate the index for the nextelement.

For example, measurement point P_(m7) on measurement scan line L_(m7) isused to correct for the aberration at element E₇ and current focal pointP_(c). To approximate the GAT™ index for element E₆ at position E₆, scanline L_(m7) is taken as a first estimate. A line c is determinedperpendicular to scan line L_(m7) at measurement point P_(m7). A line bis determined through element position E₆ and current focal point P_(c).The intersection P'_(m6) of line c and line b approximates theintersection of line b and measurement curve a (the locus of points atwhich measured aberration correction values are available). Scan lineL_(m6) on which P'_(m6) lies is determined and used as an estimate forthe GAT™ function for element E₆. This is expressed as:

    G(E.sub.6 ;L.sub.c,D.sub.c ;D.sub.m)≈L.sub.m6      (21)

A new point P_(m6), on scan line L_(m6) but exactly at the measurementrange D_(m) (i.e., on measurement curve a), is then determined. Thisscan line L_(m6) may be used in place of L_(m7) as a new first estimateto further approximate the GAT™ index for element E₆, and the stepdescribed above may be iterated as necessary to obtain an estimate ofthe GAT™ index to arbitrary accuracy. When a satisfactory estimate ofthe GAT™ index for element E₆ has been obtained, it is used as astarting point in the subsequent calculation of the GAT™ function forthe next adjacent element E₅.

4. Special Cases of the GAT™ Function

Obtaining aberration correction values using the GAT™ index functioninvolves special cases which must be considered. First, an assumptionwas made that measurement points P_(mj) and correction point P_(c) lieentirely outside aberrating region G-201 in FIG. 6a. When this is notthe case, the quality of the correction will suffer. FIGS. 12 and 13illustrate two such examples. When measurement depth D_(m) in FIG. 12lies within the aberrating region G-201, the measured aberrationcorrections will correct only for the effect of that portion ofaberrating region G-201 lying between the elements and measurement depthD_(m). When correction point P_(c) lies within aberrating region G-201,as in FIG. 13, the situation is more complicated. The focusing error dueto that portion of the aberrating region lying between the elements andcorrection point P_(c) will be accurately corrected. However, theaberration correction value will also include a component due to thatportion of aberrating region G-201 lying between correction point P_(c)and measurement depth D_(m). This region beyond D_(c) in FIG. 13 shouldideally not be included in the aberration correction, and thereforerepresents a newly introduced residual error term. However, there aretwo mitigating factors that reduce the impact of this error term. First,for focal points in the deeper half of the aberrating region G-201 (thehalf of the region furthest from the transducer element in FIG. 13), theresidual error described above will be less on average than the originalerror. Thus, imaging with the correction values should still yield a netimprovement over imaging with no correction at all. Second in manymedical ultrasound applications, imaging the body wall, the presumedsource of most aberrations, is of less interest than soft tissue. Mostof the correction points where accurate beamformation is desired willlie beyond the body wall and, hence, beyond the aberrating region G-201.

A second special case is the sensitivity of correction values generatedby the invention to certain inaccuracies in the measurement ofaberration correction profiles. Slight differences (such as steeringerrors, constant delay offsets or phase offsets) in the aberrationcorrection values measured for adjacent scan lines may result inelement-to-element delay or phase errors in aberration correctionsobtained away from a measurement depth. Even a small variation insteering error, which merely results in mild geometric distortion at ameasurement depth, may noticeably degrade beam formation away from themeasurement depth. Special care must be taken to ensure that no scanline-to-scan line bias errors are introduced into measured aberrationcorrection values, in order that the GAT™ index function work properly.

However, many aberration measurement schemes cannot measure certain"unobservable" components of each aberration correction profile. These"unobservable" components, such as steering errors, geometricallydistort the image without degrading the focus quality of an individualscan line. They correspond roughly in a planar transducer to a constantdelay offset plus a linear delay across all elements. The presentinvention operates best in combination with an adaptive beamformingultrasound system that does not introduce or attempt to correct for sucherrors. However, even when such errors exist, the aberration correctionsgenerated by the invention improve the quality of focus better than doesa single correction used at all depths.

A third special case occurs when a GAT™ index function G(.) outputs anindex value (scan line number) for which aberration correction valuesare not available. This may occur for a number of reasons. As shown inFIG. 14, near the edge of a transducer or near the edge of the scannedregion, there is no measurement focal point collinear with element E₄and the desired focal point P_(c) ; point P_(mx) is collinear with E₄and P_(c) but lies outside the set of measurement scan lines and is thusnot a true measurement point. In this case, the GAT™ index functionoutputs an index value that falls outside the range of measuredaberration correction values as illustrated by P_(mx) in FIG. 14.

Also, aberration correction values may not be available becausemeasurements were made with a limited portion of the full aperture. InFIG. 15, measurements are made at measurement points P_(m0) -P_(m26) atmeasurement depth D_(m) by an eight-element sliding aperture. That is,for each measurement point P_(m0) to P_(m26), measurements are made atthe eight elements closest to that point. Suppose the sliding aperturemoves, for example, to elements E₂ -E₉, and an aberration correctionvalue is needed corresponding to element E₉ at position E₉ and focalpoint P_(c). Measurement point P_(m4) is collinear with P_(c) and E₉.However, an aberration correction value was not measured for thecombination of element E₉ and measurement point P_(m4), because of thelimited aperture. Therefore, the GAT™ index function produces an indexvalue that does not correspond to a valid aberration correction value.

Another reason why aberration correction values may not be availableoccurs when measurement and correction occur concurrently. There may bea delay (due to processing required or other overhead) between the timea measurement scan line is fired and the time the measured aberrationcorrection values for that line become available for storage andretrieval. The measured aberration correction values for measurementscan lines recently fired may be in the process of being updated, whilethe same aberration correction values are being indexed by the GAT™function as a part of the aberration correction value for a current scanline. In some modes and in some implementations, this may not causeproblems. However, color Doppler F-mode, for example, requires severalfirings to obtain a single velocity estimate, and it is important thatthe aberration correction values used remain constant over this set offirings. In this case, aberration correction values from recently firedscan lines, which may be considered to be in flux, must not be used andare thus unavailable.

FIG. 16 illustrates how aberration correction values are updated. FIG.16 shows the space (the set of elements and measurement scan lines) overwhich correction values may be measured as a rectangle, defined by afirst and last scan line number D and D' and a first and last elementnumber E and E'. Scan line number is on the vertical axis, and may takeon any value. The element number is on the horizontal axis. Thus, thetwo horizontal lines D and D' and two vertical lines E and E' define theboundaries of "scan line"-"element" space. The various lettered linesshow boundaries in this space. Lines A and A' mark the first and lastdefined scan lines of a "pan box," a limited subset of scan lines beinggenerated in each imaging frame. Lines B and B' show the end elements ofan active aperture at a measurement depth. Lines C and C' define a"lockout region" about the current scan line in which measuredaberration corrections may be in flux or are being updated. The hatchedareas of FIG. 16 define elements and scan lines for which correctionvalues have been measured during current and previous frame times. Aframe time is considered to be the period of time between transmitting afirst scan line in a particular display format and receiving the lastscan line in the display format.

When a GAT™ index function selects a scan line number value that,coupled to the element to be corrected, corresponds to point F, in FIG.16, which is outside the region for which measured aberration correctionvalues exist, several methods can be used to estimate an aberrationcorrection value. In the preferred embodiment, the aberration correctionvalue for the same element and the closest available scan line, (shownas point F') is used. For example, F' as shown in FIG. 16, couldcorrespond to using measurement point P_(m10) to correct for P_(c) fromelement E₉ in FIG. 15. Otherwise, the aberration correction for the samescan line and the closest available element (shown as point F"), isused. For example, F" as shown in FIG. 16, could correspond to usingmeasurement point P_(m4) from element E₆ to correct for P_(c) fromelement E₉ in FIG. 15.

The above approximate corrections, used where measured correction valuesare not available, are preferable to no correction at all. However, someimage degradation, relative to an image formed with perfectly correctedbeams, will still occur. Measured correction values for points (element,scan line) far outside the region of available data will be most oftenrequested for focal points at the edges of an image and away from ameasurement depth. Therefore, image degradation resulting fromincomplete measurements occurs largely at the edges of an image and isusually away from the primary region of interest.

D. Apparatus

This section describes an apparatus for the storage and retrieval ofaberration correction values. In the preferred embodiment, correctionsare stored for subarrays of four elements. In alternative embodiments,larger or smaller subarrays or even single elements, may be used inplace of four element subarrays.

1. Ultrasound Digital Transmit and Receive Beamformer Interfaces toFocusing Control System

FIG. 17 illustrates the interface between adaptive focusing controlsystem G-100 and digital receive beamformer system R-100, beamformercentral control system C-104 and digital transmit beamformer systemT-102 shown in FIG. 2a. Each subarray summer R-126 in basebandmulti-beam processor R-125 outputs a subarray signal S_(i) on one of thedata paths G-503 to an aberration value estimator G-502 in adaptivefocusing control system G-100. Aberration value estimators G-502 areresponsible for measuring aberration correction values. These measuredaberration values for respective subarrays are then written across datapath G-506 to adaptive focus processor G-505. Adaptive focus processorG-505 then completes additional processing of the measured aberrationcorrection values before writing to delay table and interpolator G-509,preferably organized by subarray, scan line, and range. Each transmit orreceive beamformation operation initiates delay index table andinterpolator G-508, which outputs a scan line index value, preferablygenerated by the abovedescribed method, on line G-508a to delay tableand interpolator G-509. Delay table and interpolator G-509 then outputsa profile of measured or interpolated measurement aberration correctionvalues on line G-510 to beamformer central control system C-104. Thereceive beamformer aberration correction value profile is then summed,represented by summer G-515, with the receive dynamic focus delayprofile on data path G-517, and element delay values from the profileoutput on data path G-512 to each receiver's local multi-channelprocessor control C-210 in digital receive beamformer system R-100. Thetransmit beamformer aberration correction value profile on data pathG-510 is summed, through summer G-514, with the transmit focus delayprofile on data path G-518, and element delay values from the profileoutput on data path G-513 to each transmitter's local multi-channelprocessor control C-125 in digital transmit beamformer system T-102.

2. Adaptive Focusing Control System

FIG. 18 illustrates the implementation of adaptive focusing controlsystem G-100, shown in FIG. 17, according to the present invention. Alisting and description of each of the major components is discussed inSections a-h, with an overall description of the operation provided inSection V.C.3 below.

a. Adaptive Focus Processor

Adaptive focus processor G-505 communicates with many of the componentsshown in FIG. 18. In particular, adaptive focus processor G-505 controlsthe flow of information between aberration value estimators G-502illustrated in FIG. 17 and components in FIG. 18. Adaptive focusprocessor G-505 transfers data/control information to and fromaberration value estimator G-502 over data path G-506. In addition,adaptive focus processor G-505 communicates with GAT™ index table G-600on control path G-657, GAT™ high/low table G-602 on control path G-657,and GAT™ delay table G-605 on control path G-652. Adaptive focusprocessor G-505 also communicates directly with aberration valueestimators on address and data path G-506.

Adaptive focus processor G-505 is also responsible for knowing whichtransducer elements are associated with each subarray and thus whichsubarray results are to be ignored during various modes. For example,adaptive focus processor G-505 should know whether the ultrasoundimaging system is in a sliding aperture configuration.

Adaptive focus processor G-505 also initializes the GAT™ index tableG-600, GAT™ high/low table G-602 and GAT™ delay table G-605. GAT™ indextable G-600 can be initialized by 1) adaptive focus processor G-505calculating the appropriate GAT™ index values from GAT™ functions; 2)transferring predetermined GAT™ index values from an external memorylocation, such as read only memory; or 3) a combination of transferringpredetermined GAT™ index values and performing additional interpolation.

During scan geometry format changes and transmit focal depth changes,adaptive focus processor G-505 computes or enters the indices into GAT™index table G-600. During B-mode operation, adaptive focus processorG-505 is involved in real-time computation of measured aberrationcorrection values. Adaptive focus processor G-505 reads the aberrationcorrection values output from the aberration value estimators G-502,processes them, and outputs the results to GAT™ delay table G-605.

In the preferred embodiment, GAT™ delay table G-605 is continuouslyupdated only for B-mode scanning and is also updated during B-mode scanlines of mixed modes, such as B-and-F or B-and-D modes. Non-B-mode scanlines of mixed modes may still retrieve aberration correction valuesfrom the GAT™ delay table G-605 in order to correct these non-B-modescan lines. However, the GAT™ delay table G-605 will only be updatedwith data from B-mode scan lines. In an alternative embodiment, GAT™delay table G-605 could be partitioned and updated with correctionvalues acquired during another scanning mode, such as color DopplerF-mode, with these delay correction values kept in a separate partitionfrom that of the B-mode correction values.

b. GAT™ Index Table

FIG. 19 is a detailed illustration of GAT™ index table G-600 shown inFIG. 18.

GAT™ index table G-600 consists of a bit mapping random access memory(RAM). GAT™ index table G-600 is partitioned into sections indexed by abase address B_(n). Each of these sections contains a GAT™ index for Nreference scan lines (where N is a power of 2) and M subarrays, e.g.M=16, 32 or 64. The GAT™ index table G-600 allows a mapping from thecurrent subarray, current scan line, current focal depth, and currentscan geometry format to the equivalent measurement scan line number interms of an aberration value, as seen by the current subarray, whenfocused at a selected measurement depth.

Address information can be split among three input values: 1) baseaddress, 2) correction scan line number, and 3) subarray number. Baseaddress can be further split into four values: 1) transducer number(separate correction and GAT index for e.g. 2 separate transducers ispossible), 2) scan geometry format, 3) correction depth, and 4)measurement depth.

c. GAT™ Index Interpolator

FIG. 18 illustrates an index interpolator G-601 coupled to GAT™ indextable G-600 and index modifier block G-603 by data paths G-691 andG-651, respectively. Index interpolator G-601 is responsible forinterpolating between index values corresponding to reference linesoutput from GAT™ index table G-600. A scan line interpolation parameterα_(L) is supplied from a focusing control C-132 on line G-655 to indexinterpolator G-601. The scan line interpolation parameter α_(L) iscalculated according to equation (13) by the focusing control C-132 inorder to allow index interpolator G-601 to obtain GAT™ index values forthe current line by interpolating between GAT™ index valuescorresponding to a pair of reference scan lines, as shown in equation(12). The interpolated GAT™ index value is then output on data pathG-651 to GAT™ index modifier block G-603.

d. GAT™ High/Low Table

FIG. 20 is a detailed illustration of GAT™ high/low table G-602 in FIG.18. GAT™ high/low table G-602 contains the low scan line and high scanline numbers for all subarrays at a selected measurement depth. The highand low scan line values represent the boundaries of the active aperturein GAT™ delay table G-605. High scan line and low scan line values inGAT™ high/low table G-602 are illustrated by lines B and B' in FIG. 16.

GAT™ high/low table G-602 consists of a bit mapping RAM. GAT™ high/lowtable G-602 is partitioned into sections by base addresses B_(n).Finally, high and low scan line values are written by adaptive focusprocessor G-505 on control path G-682 to GAT™ high/low table G-602. Highand low scan line values are addressed by either adaptive focusprocessor G-505 or focusing control C-132 by way of multiplexer G-610,which is controlled by signals from adaptive focus processor G-505.

e. GAT™ Index Modifier Block

Index modifier block G-603 is responsible for modifying GAT™ indexvalues on data path G-651 based upon inputs from GAT™ high/low tableG-602 and parameters from focusing control C-132 on control path G-681.GAT™ index modifier block G-603 will determine whether an index value ondata path G-651 falls within the valid index values for the selectedimaging format. FIG. 16 illustrates the valid aberration correctionvalues defined by lines A-A' and B-B' for a given set of defined scanlines and transducer elements. As described operationally below,numerous methods may be used to obtain aberration correction values whena requested GAT™ index falls outside of the valid aberration region. Ifa correct index value cannot be obtained, or the correspondingaberration value is in lockout region C-C' in FIG. 16, a signal may begenerated on control path G-650 to set the aberration value output ondata path G-670 to zero. Finally, an index value which may have beenmodified is output on data path G-661 in order to obtain a validaberration correction value in GAT™ delay table G-605.

f. GAT™ Delay Table

FIG. 21 is a detailed illustration of GAT™ delay table G-605 shown inFIG. 18.

GAT™ delay table G-605 consists of a bit mapping RAM that contains delaydata expressed in terms of some nominal period. In the preferredembodiment, the delay is expressed as a signed quantity with resolutionof a small fraction of the period. For situations where the frequencychanges, such as adjustable frequency or in the presence ofdownshifting, delay values for all scan lines in GAT™ delay table G-605will be expressed in terms of the same nominal period. Adjustablefrequency is more fully described in the above-referenced andincorporated co-pending patent application entitled: METHOD ANDAPPARATUS FOR ADJUSTABLE FREQUENCY SCANNING IN ULTRASOUND IMAGING.

GAT™ delay table G-605 is partitioned into sections by base addressB_(n). In each section, delay values are stored for a subarray oftransducer elements at a measurement depth.

g. GAT™ Delay Interpolator

Delay interpolator G-604 in FIG. 18 calculates individual transducerelement aberration correction delay values from subarray aberrationcorrection delay values output from GAT™ delay table G-605 on data pathG-670. GAT™ delay interpolator G-604 uses signals on control path G-650from index modifier block G-603 and element interpolation parameterα_(E) on control path G-659 derived from focusing control C-132. Timedelays for individual elements are obtained by interpolating between twooutput subarray aberration correction values T_(A) and T_(B) output ondata path G-670. For example, subarray aberration correction value T_(A)corresponds to subarray A consisting of elements E₀ -E₃. Adjacentsubarray aberration value T_(B) corresponds to adjacent subarray Bconsisting of elements E₄ -E₇. The time delays T_(E2) . . . T_(E5) forthe elements E₂, E₃, E₄, and E₅ are determined as follows:

    T.sub.Ei =α.sub.Ei T.sub.A +(1-α.sub.Ei)T.sub.B i=2, . . . , 5(22)

where α_(Ei) is the value output on control path 659 from focusingcontrol C-132.

In another embodiment, the delay interpolator G-604 in combination withthe index modifier block G-603 (which provides an interpolationparameter) could incorporate the function of interpolating the delaysamples in scan line number, calculating aberration correction valuesfor scan lines between those actually measured.

Finally, if index modifier block G-603 determines that an index valuefor GAT™ delay table G-605 is invalid, a signal will be asserted oncontrol path G-650 to delay interpolator G-604 to set the delay valueson data path G-660 to zero.

3. Operational Description of GAT™ Apparatus

The following describes the operation of a GAT™ apparatus, asillustrated in FIG. 18. At the start of scanning in a given scangeometry with some known set of measurement foci, adaptive focusprocessor G-505 calculates GAT™ index values for each of severalreference scan lines. Adaptive focus processor G-505 stores these valuesin GAT™ index table G-600. Adaptive focus processor G-505 also storesvalues in GAT™ high/low table G-602 for the selected measurement scangeometry for which measured delay values are available.

Aberration value estimators G-502 in FIG. 17 measure initial estimatesof aberration correction values at the selected measurement foci, andtransfer these estimates to adaptive focus processor G-505. Adaptivefocus processor G-505 loads the estimate into GAT™ delay table G-605.

Alternately, no initial estimates may be made by aberration valueestimators G-502. In this case, predetermined aberration correctionvalues will be downloaded from memory within an adaptive ultrasoundimaging system to the adaptive focus processor G-505 and then written toGAT™ delay table G-605.

To obtain aberration correction values at a selected depth and currentscan line number, index values are read from GAT™ index table G-600 ofFIG. 18 for the reference scan lines above and below the selected scanline number for the selected focus depth and for each subarray. Allthese values are supplied to GAT™ index table through multiplexer G-609from focusing control C-132 on data path 655. Multiplexer G-609 iscontrolled by inputs on control path G-690 from focusing control C-132.

GAT™ index interpolator G-601 then interpolates between the two GAT™index values at the reference scan lines to obtain an index value at thecurrent scan line. As mentioned above, the GAT™ index values (at thereference scan lines) stored in the GAT™ index table G-600 may refer tonon-existent scan lines. For example, negative scan line numbers may beoutput, as illustrated (by P_(mx)) in FIG. 14. Index interpolator G-601is able to interpolate between two existing scan line numbers, betweenan existing and a non-existing scan line number, or between twonon-existing scan line numbers and still produce a consistent result.

A GAT™ index value obtained for each subarray is transferred on datapath G-651 to index modifier block G-603. At the same time, for eachsubarray, the high and low scan lines for which an aberration value isavailable (given a scan geometry format and measurement depth) is readon control path G-662 from GAT™ high/low table G-602 and passed to theindex modifier block G-603. At the same time, the first and last scanlines fired in the current frame are passed to the index modifier blockG-603 via control path G-681.

The index modifier block G-603 outputs to multiplexer G-611 on data pathG-661, the available measurement scan line closest to the requested GAT™index value. That is, of all the measurement scan lines for which acorrection value is known to exist for the selected subarray, the indexmodifier block selects the scan line closest to the requested GAT™ indexvalue. If a correction value is not known for any scan line number forthe selected subarray, the index modifier block G-603 generates a signalon control path G-650. Delay interpolator G-604 will then disablecorrections (by outputting zero values) for the selected subarray outputfrom the GAT™ delay table G-605.

In an alternative mode, GAT™ index modifier block G-603 may alsooverride a GAT™ index value output on data path G-651, selecting for allsubarrays either 1) the current scan line, 2) the low boundary scan lineof the lockout region C-C', or 3) the high boundary scan line of thelockout region C-C'. The low and high boundary scan lines of lockoutregion C-C' are scan lines immediately outside the lockout region.)These modes may be used at or near the measurement foci. Selecting thecurrent line near the measurement foci essentially disables the GAT™near the measurement foci so that the GAT™ does not interfere withadaption. Selecting the lowest line outside the lockout region may beused to support adaption, as described in the above-referenced andincorporated patent application entitled: METHOD AND APPARATUS FORREAL-TIME, CONCURRENT ADAPTIVE FOCUSING IN AN ULTRASOUND BEAMFORMERIMAGING SYSTEM.

In an alternate mode, the index modifier block may exclude scan lineswithin a lockout region C-C'. When a scan line within lockout regionC-C' is selected, the index modifier block may return either 1) thecurrent scan line, 2) the low boundary scan line of lockout region C-C',or 3) the high boundary scan line of lockout region C-C'. Selecting oneof the boundary scan lines outside the lockout region may be done, forexample, in F-mode, to keep applied aberration correction values fromchanging during the course of the several ultrasound firings required toobtain one F-mode Doppler-processed scan line.

In an alternate mode, the index modifier block may add an offset to theindex output on data path G-651, before carrying out subsequent steps ofdetermining the closest valid scan line number and excluding scan linenumbers in the lockout region. For points close to the measurement focusthis keeps the returned index out of the lockout region without causingany discontinuity in the applied correction profile (which would resultfrom one subarray using a correction value from one edge of the lockoutregion and an adjacent subarray element using a correction value fromanother edge of the lockout region).

The index modifier block may be programmed to simply disablecorrections, by generating a signal on control path G-650 to delayinterpolator G-604. This may be done, for example, on the first few scanlines or the first frame after a scan geometry format change to preventcorrection using invalid data.

In an alternate mode, if the index returned by the above steps differsfrom the nominal value output on data path G-651 by more than aspecified amount, the index modifier block may disable (zero) thecorrections sent to the selected subarray. For example, if for someelement, scan line number 12 is output on data path G-651, and the firstvalid scan line returned by the above steps is scan line number 100,then the correction value measured for scan line number 100 may have nocorrelation to the correction value needed for scan line number 12. Inthis case, it is better to disable correction than to use invalidcorrection values.

In alternate modes, GAT™ index modifier block G-603 may take intoaccount the age of correction values. In FIG. 16, the various hatchedregions indicate those scan lines and elements or subarrays for whichcorrection values have been measured. Within this region, a furtherdistinction may be made between the ages of the measured aberrations.Because correction values are measured and stored sequentially with eachfiring, some of the correction values in GAT™ delay table G-605 willhave been measured in the current scanning frame and some in theprevious frame. In FIG. 16, the order of scan line firing is from top tobottom. Thus, correction values for those scan lines above the currentscan line will have been acquired during the current frame, andcorrection values for those scan lines below the current scan line willhave been acquired during the previous frame. Within the shaded regionA-C' in FIG. 16, the age of the correction values progresses smoothly,with the oldest estimates at the top and the newest at the bottom. Indexmodifier block G-603 may, in some modes, disable correction when theindex returned by the above steps is older than the current scan line bysome specified amount.

An index value determined by the index modifier block G-603 is thenoutput on data path G-661 to multiplexer G-611 in order to address GAT™delay table G-605. Multiplexer G-611 is controlled by a clock signalwhich is divided by two on control path G-700. A corresponding subarrayaberration correction value is output on data path G-670 to delayinterpolator G-604. Delay interpolator G-604 converts two subarrayaberration correction values to aberration correction values forindividual transducer elements according to equation (22). Finally, theaberration corrections are transferred on data path G-510 to summersG-515 and G-514, illustrated in FIG. 17, where they are used to correctfocusing for both transmit and receive ultrasound beamformers.

Accordingly, the GAT™ function may be used to generate delay correctionson both transmit and receive. In other words, aberration correctionvalues may be used to correct focus for an ultrasound firing, and thereturned signals from the same ultrasound firing may then be used tomeasure aberration correction values.

Adaptive focus processor G-505 may also interleave writing newlymeasured aberration correction values to GAT™ delay table G-605 whilereading out values from GAT™ delay table G-605 for correcting focus onsubsequent scan lines.

E. Conclusion

An ultrasound system having a GAT™ apparatus and method is disclosed.The GAT™ function allows for aberration correction values measured alongscan lines, preferably at a transmit focus depth D_(m) to be transformedand used for any focal points P_(c) in an imaged subject. Thetransformation allows for the use of aberration correction values at allpoints in an image without requiring additional measurements, specialscan geometries, or additional frame time.

The foregoing description of the preferred embodiments of the presentinvention has been provided for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise forms disclosed. Obviously, many modificationsand variations will be apparent to practitioners skilled in the art. Theembodiments where chosen and described in order to best explain theprinciples of the invention and its practical applications, therebyenabling others skilled in the art to understand the invention forvarious embodiments and with the various modifications as are suited tothe particular use contemplated. It is intended that the scope of theinvention be defined by the following claims and their equivalents.

What is claimed is:
 1. A method for providing aberration correctionvalues in an ultrasonic imaging system to compensate for propagationaberrations in a medium, said ultrasonic system comprising a pluralityof transducer elements organized into a plurality of transducersubarrays, comprising the steps of:storing a first plurality ofaberration correction values, each aberration correction valuecorresponding to a respective one of a plurality of transducer subarraysand a respective one of a plurality of first locations in said medium;and selecting for a second location in said medium a second plurality ofaberration correction values from the first plurality of aberrationcorrection values wherein at least two of the selected aberrationcorrection values correspond to distinct first locations, and whereinsaid second location is at a distinct depth from the first locations. 2.The method of claim 1, wherein the storing and selecting occurconcurrently with imaging.
 3. The method of claim 1, wherein the firstplurality of aberration correction values are delay values.
 4. Themethod of claim 1, wherein the first plurality of aberration correctionvalues are phase values.
 5. The method of claim 1, wherein the firstplurality of aberration correction values are amplitude values.
 6. Themethod of claim 1, wherein the aberration correction values are selectedfrom the group consisting of at least two of delay, phase and amplitudevalues.
 7. The method of claim 1, wherein the first plurality ofaberration correction values correspond to a first frequency and thesecond plurality of aberration correction values correspond to a secondfrequency.
 8. The method of claim 1, wherein the second plurality ofaberration correction values are delay values.
 9. The method of claim 1,wherein the second plurality of aberration correction values are phasevalues.
 10. The method of claim 1, wherein the second plurality ofaberration correction values are amplitude values.
 11. The method ofclaim 1, wherein the second plurality of aberration correction valuesare selected from the group consisting of at least two of delay, phaseand amplitude values.
 12. The method of claim 1, wherein the aberrationcorrection values are measured aberration correction values.
 13. Themethod of claim 1, wherein the first location is a measurement location.14. The method of claim 13, wherein the measurement location is at orabout a transmit focal point.
 15. The method of claim 1, wherein thesecond location is a selected location.
 16. The method of claim 15,wherein the selected location is at or about a receive focal point. 17.The method of claim 1, wherein the selecting step includes generatingindex values associated with the second plurality of aberrationcorrection values.
 18. The method of claim 1, wherein the storing stepobtains aberration correction values from at least one imaging scanline.
 19. The method of claim 18, wherein the scan lines are selectedfrom the group consisting of a B-mode scan lines and color Doppler modescan lines.
 20. The method of claim 1, wherein the selecting stepobtains aberration correction values for at least one scan line.
 21. Themethod of claim 20, wherein the scan lines are selected from the groupconsisting of B-mode scan lines, color Doppler mode scan lines, D-modescan lines, and M-mode scan lines.
 22. The method of claim 1, wherein atleast one of the plurality of transducer subarrays comprises a singletransducer element.
 23. The method of claim 1, wherein at least one ofthe plurality of transducer subarrays comprises at least two transducerelements.
 24. The method of claim 1, wherein the storing and selectingcorrespond to a first zone of a plurality of zones.
 25. The method ofclaim 1 wherein the ultrasound imaging system includes an index tablecoupled to an aberration correction value table storing the firstplurality of aberration correction values and the step of selecting isfurther defined by the steps of:generating a plurality of index valuesfor the index table, each index value corresponding to a transducersubarray, a scan line number, a correction depth, and a measurementdepth; selecting an index value from the plurality of index values basedon a selected transducer subarray, a selected scan line number and aselected correction depth; and retrieving the second plurality ofaberration correction values, responsive to a plurality of selectedindex values, from the aberration correction value table.
 26. The methodof claim 25, wherein the selecting and retrieving occurs concurrentlywith imaging.
 27. The method of claim 25, wherein the step of selectingan index value is further defined by the step of:interpolating between afirst index value and a second index value.
 28. The method of claim 25,wherein the step of retrieving is further defined by the stepof:interpolating between a first aberration correction value and asecond aberration correction value.
 29. The method of claim 25, whereinthe step of retrieving is further defined by the step of:interpolatingbetween a first aberration correction value and a second aberrationcorrection value to affect finer scan line spacing.
 30. The method ofclaim 25, wherein the step of retrieving is further defined by the stepof:interpolating between a first aberration correction value and asecond aberration correction value to affect finer subarray spacing. 31.The method of claims 1, 2, 6, 7, 11, 17, 20, 24, or 25 wherein theselected correction values vary by scan line for at least some scanlines.
 32. The method of claims 1, 2, 6, 7, 11, 17, 20, 24, or 25wherein at least one of the plurality of transducer subarrays comprisesfour transducer elements.
 33. The method of claims 1, 2, 6, 7, 11, 17,20, 24, or 25 wherein at least four of the selected aberrationcorrection values for a single second location correspond to respectivedistinct first locations.
 34. The method of claim 1 wherein at least oneselected correction value corresponds to a first location at or about anextension of a line from the corresponding subarray to the secondlocation.
 35. A method for providing aberration correction values in anultrasonic imaging system having a plurality of transducer subarrayscomprising the step of:generating a plurality of index values; storingsaid index values in an index table; measuring a plurality of aberrationcorrection values; storing said plurality of aberration correctionvalues in an aberration correction value table coupled to said indextable; selecting a plurality of index values from said index table basedon a selected transducer subarray and a selected correction location;and retrieving a second plurality of aberration correction values,responsive to the plurality of selected index values, from theaberration correction value table.
 36. The method of claim 35, whereinthe aberration correction values are measured aberration correctionvalues.
 37. The method of claim 35, wherein the aberration correctionvalues correspond to at least one imaging scan line.
 38. The method ofclaim 37, wherein the scan lines are selected from the group consistingof B-mode scan lines and color Doppler mode scan lines.
 39. The methodof claim 35, wherein the selecting step obtains aberration correctionvalues for at least one imaging scan line.
 40. The method of claim 39,wherein the scan lines are selected from the group consisting of B-modescan lines, color Doppler mode scan lines, D-mode scan lines, and M-modescan lines.
 41. The method of claim 35, wherein the step of selecting anindex value is further defined by the step of:interpolating between afirst index value and a second index value.
 42. The method of claim 35,wherein the step of retrieving is further defined by the stepof:interpolating between a first aberration correction value and asecond aberration correction value.
 43. The method of claim 35, whereinthe step of retrieving is further defined by the step of:interpolatingbetween a first aberration correction value and a second aberrationcorrection value to affect finer scan line spacing.
 44. The method ofclaim 35, wherein the step of retrieving is further defined by the stepof:interpolating between a first aberration correction value and asecond aberration correction value to affect finer subarray spacing. 45.The method of claim 35, wherein at least one of the transducer subarrayscomprises a single transducer element.
 46. The method of claim 35,wherein at least one of the transducer subarrays comprises at least twotransducer element.
 47. The method of claim 35, wherein the step ofretrieving a second plurality of measured aberration correction valuesis further defined by the step of:retrieving measured aberrationcorrection values from a subset of the aberration correction values inthe aberration correction value table.
 48. The method of claim 47,wherein the steps further include the step of:modifying the selectedindex value to correspond to an aberration correction value in thesubset of the aberration correction values in the aberration correctionvalue table.
 49. The method of claim 35 further wherein the generatingstep comprises:generating the plurality of index values for the indextable, each index value corresponding toa transducer subarray, acorrection location, and a measurement depth.
 50. The method of claims35 or 49 wherein said plurality of index values corresponds to at leastfour measurement locations.
 51. The method of claims 35 or 49 wherein atleast one of the plurality of transducer subarrays comprises fourtransducer elements.
 52. The method of claims 35 or 49 wherein at leastfour of the retrieved aberration correction values corresponding toindex values for a single selected correction location correspond to atleast four respective distinct measurement locations.
 53. A method forobtaining aberration correction values for a subject having anaberrating region using an ultrasonic imaging system wherein ultrasoundsignals are transmitted from an ultrasonic array having a plurality oftransducer subarrays to a measurement depth in the subject and reflectedtherefrom, the method comprising the steps of:measuring a first set ofaberration correction values in a first scan geometry format based onthe reflected ultrasonic signals, wherein each aberration correctionvalue corresponds to one of the plurality of transducer subarrays and ameasurement location; and using the first set of aberration correctionvalues to form a second set of aberration correction values in a secondscan geometry format different from the first scan geometry format. 54.The method of claim 53, wherein the reflected signals are used for bothmeasuring aberration correction values and imaging.
 55. The method ofclaim 53, wherein the measuring, storing and mapping occur concurrentlywith imaging.
 56. The method of claim 53, wherein at least one of thetransducer subarrays comprises a single transducer element.
 57. Themethod of claim 53, wherein at least one of the transducer subarrayscomprises at least two transducer elements.
 58. The method of claim 53wherein the first set of aberration correction values are measured at aplurality of measurement depths.
 59. The method of claim 53, wherein thestoring step obtains aberration correction values from at least oneimaging scan line.
 60. The method of claim 59, wherein the scan linesare selected from the group consisting of a B-mode scan lines and colorDoppler mode scan lines.
 61. The method of claim 53, wherein the mappingstep obtains aberration correction values for at least one imaging scanline.
 62. The method of claim 61, wherein the scan lines are selectedfrom the group consisting of B-mode scan lines, color Doppler mode scanlines, D-mode scan lines, and M-mode scan lines.
 63. The method ofclaims 53, 54, or 58 wherein at least four aberration correction valuesin said second set for a single correction values in said second set fora single correction location correspond to at least four respectivedistinct measurement locations.
 64. An ultrasonic imaging system havinga transmitter transmitting ultrasonic signals from an array oftransducer elements grouped into subarrays and a receiver receivingreflected signals, comprising:means, coupled to the receiver, formeasuring aberration correction values based upon reflected signalsreceived from measurement locations in a subject; and means, coupled tothe measuring means, for geometrically transforming the aberrationcorrection values at measurement locations to aberration correctionvalues at selected locations, wherein at least a first one of saidselected locations is different than the measurement locations, whereinat least two of the geometrically transformed aberration correctionvalues at said first selected locations correspond to distinctmeasurement locations, and wherein said first selected location is at adistinct depth from the measurement locations.
 65. The apparatus ofclaim 64, wherein the means for measuring and means for transformingoccur concurrently with imaging.
 66. The apparatus of claim 64, whereinthe means for measuring is operating in a mode selected from the groupconsisting of B-mode and color Doppler mode.
 67. The apparatus of claim64, wherein the means for transforming is operating in a mode selectedfrom the group consisting of B-mode, color Doppler mode, M-mode orD-mode.
 68. The apparatus of claim 64, wherein the means for measuringobtains aberration correction values from at least one imaging scanline.
 69. The apparatus of claim 64, wherein the means for transformingobtains aberration correction values for at least one scan line.
 70. Theapparatus of claim 64, wherein the means for transforming includes:anindex table having a plurality of index values, each index valuecorresponding to a transducer subarray, a measurement depth, acorrection depth, and a scan line number, outputting an index valueresponsive to a selected transducer subarray, a selected correctiondepth and a selected scan line number; an aberration correction valuetable, coupled to the index table, having a plurality of aberrationcorrection values, each aberration correction value corresponding to atransducer subarray, a measurement depth and a scan line number,outputting an aberration correction value responsive to the index value;and a processor, coupled to the index table, selecting the selectedtransducer subarray, the selected correction depth and selected scanline number.
 71. The apparatus of claim 70, further comprising:means,coupled to the index table, for interpolating between a first indexvalue and a second index value.
 72. The apparatus of claim 70, furthercomprising:means for interpolating between a first aberration correctionvalue and a second aberration correction value to affect finer scan linespacing.
 73. The apparatus of claim 70, further comprising:means forinterpolating between a first aberration correction value and a secondaberration correction value to affect finer subarray spacing.
 74. Theapparatus of claim 70, further comprising:means, coupled to anaberration correction value table, for interpolating between a firstaberration correction value and a second aberration correction value.75. The system of claims 64, 68, or 70 wherein at least one of theplurality of transducer subarrays comprises four transducer elements.76. The system of claims 64, 68, or 70 wherein at least four of thetransformed aberration correction values for a single selected locationcorrespond to at least four respective distinct measurement locations.77. An improved apparatus for use in an ultrasonic imaging system thatincludesan ultrasonic array of transducer elements; a transmitbeamformer, coupled to the ultrasonic array; and a receive beamformer,coupled to the ultrasonic array;the improved apparatus comprising: meansfor summing signals from the plurality of transducer elements to form aplurality of subarray signals; means, coupled to the receive beamformer,for measuring a first plurality of aberration correction values based onthe plurality of subarray signals and a measurement location; and means,coupled to the measuring means, for transforming the first plurality ofaberration correction values to a second plurality of aberrationcorrection values for selected correction locations, said selectedcorrection locations being different than the measurement locations, andwherein at least two of the selected aberration correction valuesselected for a given correction location correspond to distinctmeasurement locations.
 78. The apparatus of claim 77, wherein thetransmit beamformer transmitting, receive beamformer receiving, meansfor measuring, and means for transforming are during an imaging scanningmode.
 79. The apparatus of claim 77, wherein the received ultrasoundbeam is used both for measuring aberration correction values andimaging.
 80. The apparatus of claim 77, wherein the means for measuringand means for transforming occur concurrently with imaging.
 81. Theapparatus of claim 77, wherein the means for measuring obtainsaberration correction values from at least one imaging scan line. 82.The apparatus of claim 81, wherein the imaging scan lines are selectedfrom the group consisting of B-mode scan lines and color Doppler modescan lines.
 83. The apparatus of claim 77, wherein the means fortransforming obtains aberration correction values for at least one scanline.
 84. The apparatus of claim 83, wherein the scan lines are selectedfrom the group consisting of B-mode scan lines, color Doppler mode scanlines, D-mode scan lines, and M-mode scan lines.
 85. The apparatus ofclaim 77, wherein the means for transforming includes:an index tablehaving a plurality of index values, each index value corresponding to atransducer subarray, a measurement depth, a correction depth, and a scanline number, outputting an index value responsive to a selectedtransducer subarray, a selected correction depth, and a selected scanline number; an aberration correction value table, coupled to the indextable, having a plurality of aberration correction values, eachaberration correction value corresponding to a measurement depth, scanline number, transducer subarray, outputting an aberration correctionvalue responsive to the index value; and a processor, coupled to theindex table, selecting the selected transducer subarray, a selectedcorrection depth, and a selected scan line number.
 86. The apparatus ofclaim 85, further comprising:means, coupled to the index table, forinterpolating between a first index value and a second index value. 87.The apparatus of claim 85, further comprising:means for interpolatingbetween a first aberration correction value and a second aberrationcorrection value to affect finer scan line spacing.
 88. The apparatus ofclaim 85, further comprising:means for interpolating between a firstaberration correction value and a second aberration correction value toaffect finer subarray spacing.
 89. The apparatus of claim 85, furthercomprising:means, coupled to an aberration correction value table, forinterpolating between a first aberration correction value and a secondaberration correction value.
 90. The apparatus of claim 85, wherein themeans for transforming further includes:means, coupled to the indextable and aberration correction value table, for modifying the indexvalues to correspond to aberration correction values in a subset of theaberration correction value in the aberration correction value table.91. The apparatus of claim 77 wherein said first plurality of aberrationcorrection values for a single correction location correspond to atleast four distinct measurement locations.
 92. The apparatus of claim 77wherein at least four of the selected aberration correction values for acorrection location correspond to at least four respective distinctmeasurement locations.
 93. The apparatus of claim 77 wherein signalsfrom four transducer elements are summed to form at least one of thesubarray signals.
 94. The invention of claims 18, 20, 37, 39, 59, 61,68, 69, 81, or 83 wherein the scan lines are from a scan geometry formatselected from a group consisting of Vector® and curved Vector®.
 95. Theinvention of claims 18, 20, 37, 39, 59, 61, 68, 69, 81, or 83 whereinthe scan lines are from a scan geometry format selected from a groupconsisting of sector, linear, curved linear, steered linear, and steeredcurved linear.